selftests/bpf: Test refinement of single-value tnum

selftests/bpf: Test refinement of single-value tnum

This patch introduces selftests to cover the new bounds refinement
logic introduced in the previous patch. Without the previous patch,
the first two tests fail because of the invariant violation they
trigger. The last test fails because the R10 access is not detected as
dead code. In addition, all three tests fail because of R0 having a
non-constant value in the verifier logs.

In addition, the last two cases are covering the negative cases: when we
shouldn't refine the bounds because the u64 and tnum overlap in at least
two values.

Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Link: https://lore.kernel.org/r/90d880c8cf587b9f7dc715d8961cd1b8111d01a8.1772225741.git.paul.chaignon@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>